Transformer wires when used within dry type transformers generally comprise at least one rectangular wire covered first with a layer of thin asbestos paper followed by a covering of synthetic fibre material for toughness. The covered wire is then provided with a varnish overcoat to give further mechanical protection and abrasion resistance when the covered wire is wound into a transformer coil.
The inherent weak mechanical properties of the asbestos paper require the use of the fibre to keep the asbestos from tearing and exposing the rectangular wire. The varnish overcoat is needed to provide the necessary degree of abrasion resistance to the coated wire so that the coated wire can be passed through guides in the coil making process without catching and tearing the asbestos and fibre coating.
The use of the synthetic strengthening fibre and the provision of a varnish overcoat contributes to the material expenses involved in transformer manufacture. The purpose of this invention is to provide an asbestos paper insulation that has the required degree of toughness and lubricity to withstand the coil winding process without the synthetic fibre and varnish overcoat.